This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-215318, filed Jul. 29, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a high breakdown voltage semiconductor device and, more specifically, to a technique of reducing the resistance of a drift region of a high breakdown voltage MOSFET.
A high breakdown voltage semiconductor device, such as a high breakdown voltage MOSFET and IGBT, has a drift region whose impurity concentration is relatively low in order to obtain a high breakdown voltage. If a high voltage is applied to the element in an off state, the drift region is depleted and the element withstands the high voltage. Since the drift region is low in impurity concentration, it increases in resistance and has a large share of on-resistance of the element. As the breakdown voltage heightens, the drift region increases in resistance; accordingly, the on-resistance of the element increases.
A high breakdown voltage MOSFET, which has the structure as shown in FIG. 20 to decrease the resistance of a drift region, is proposed in T. Fujihira, xe2x80x9cTheory of Semiconductor Superjunction Devices,xe2x80x9d Jpn. J. Appl. Phys., Vol. 36 (1997), pp 6254-6262. In this MOSFET, a drift region 43 is formed between a p-type body region 41 and an n+-type drain region 42 in order to obtain a high breakdown voltage. The drift region 42 is constituted of p-type layers 44 and n-type layers 45 which are arranged alternately at small pitches in the channel width direction. If a high voltage is applied between the source and drain so as to make the drain positive, a depletion layer expands not only from both a pn junction between the p-type body region 41 and n-type layers 45 and a pn junction between a p-type layer 46 and the n-type layers 45, but also from a pn junction between the p-type layers 44 and n-type layers 45. For this reason, even though the impurity concentration of the n-type layers 45 is set higher than that of the drift region of a normal high breakdown voltage MOSFET, the n-type layers 45 can be depleted and the breakdown voltage can be maintained. With the structure of the MOSFET shown in FIG. 20, therefore, the resistance of the drift region can be lowered by increasing the impurity concentration of the n-type layers 45.
However, in order to achieve the above advantage, the impurity concentration of the n-type layers 45 has to be at least two times as high as that of the normal high breakdown voltage MOSFET, provided that the Pxe2x88x92 and n-type layers 44 and 45 have the same width a. In order to prevent the breakdown voltage from lowering in this state, the width a need to be reduced and the depth b of the layers 44 and 45 need to be increased to some extent. For example, the depth b should be 2 xcexcm at the minimum when the width a is 1 xcexcm. To reduce the on-resistance to half by the same breakdown voltage as that of the normal MOSFET, the depth b should be about 4 xcexcm at the minimum when the width a is 1 xcexcm. Such a structure is difficult to achieve by the current manufacturing technique and, if it is done, its manufacture process will be complicated and increased in cost.
As described above, there is a problem in which the conventional superjunction elements are difficult to manufacture in actuality.
The present invention has been developed in consideration of the above situation and its object is to provide a high breakdown voltage semiconductor device which is easy to manufacture and low in on-resistance.
According to a first aspect of the present invention, there is provided a high breakdown voltage semiconductor device formed on a high resistance semiconductor layer, comprising a plurality of trenches formed like a stripe in a drift region in substantially parallel with a current flowing direction, an insulation film formed on a side and a bottom of each of the trenches, and a high resistance film buried into each of the trenches with the insulation film interposed therebetween, wherein the high resistance film is connected to one of source and gate electrodes directly or through a resistor near a source-end portion of each of the trenches, and the high resistance film is connected to a drain electrode directly or through a resistor near a drain-end portion of each of the trenches.
In the high breakdown voltage semiconductor device according to the first aspect of the present invention, the high resistance film may be formed on one of a semi-insulating polycrystalline silicon and polysilicon.
According to a second aspect of the present invention, there is provided a high breakdown voltage semiconductor device formed on a high resistance semiconductor layer of a first conductivity type, comprising a drift region of a second conductivity type selectively formed on a surface of the high resistance semiconductor layer, a plurality of trenches formed like a stripe in the drift region in substantially parallel with a current flowing direction, an insulation film formed on a side and a bottom of each of the trenches, and a high resistance film buried into each of the trenches with the insulation film interposed therebetween, wherein the high resistance film is connected to one of source and gate electrodes directly or through a resistor near a source-end portion of each of the trenches, and the high resistance film is connected to a drain electrode directly or through a resistor near a drain-end portion thereof.
In the high breakdown voltage semiconductor device according to the second aspect of the present invention, the high resistance film may be formed on one of a semi-insulating polycrystalline silicon and polysilicon.
According to a third aspect of the present invention, there is provided a high breakdown voltage semiconductor device including a first high breakdown voltage MOSFET, a second high breakdown voltage MOSFET, a resistor, comprising a plurality of trenches formed like a stripe in a drift region of the first high breakdown voltage MOSFET, in substantially parallel with a current flowing direction, an insulation film formed on a side and a bottom of each of the trenches, and a high resistance film buried into each of the trenches with the insulation film interposed therebetween, wherein the first high breakdown voltage MOSFET and the second high breakdown voltage MOSFET have a common source electrode and a common gate electrode, and one end of the high resistance film is connected to the common gate electrode, while another end thereof is connected to a drain electrode of the second high breakdown voltage MOSFET and one end of the resistor.
In the high breakdown voltage semiconductor device according to the third aspect of the present invention, the high resistance film may be formed on one of a semi-insulating polycrystalline silicon and polysilicon.
In the high breakdown voltage semiconductor device according to the third aspect of the present invention, the first high breakdown voltage MOSFET and the second high breakdown voltage MOSFET may be formed in a common semiconductor layer. The high resistance film may be formed on one of a semi-insulating polycrystalline silicon and polysilicon.
According to a fourth aspect of the present invention, there is provided a high breakdown voltage semiconductor device formed on a high resistance semiconductor layer, comprising, a plurality of trenches formed like a stripe in a drift region in substantially parallel with a current flowing direction, an insulation film formed on a side of each of the trenches, and a high resistance film buried into each of the trenches, wherein the high resistance film is connected to one of source and gate electrodes directly or through a resistor near a source-end portion of each of the trenches, and the high resistance film is connected to a drain region near a drain-end portion of each of the trenches.
In the high breakdown voltage semiconductor device according to the fourth aspect of the present invention, the high resistance film may be formed on one of a semi-insulating polycrystalline silicon and polysilicon.
In the high breakdown voltage semiconductor device according to the present invention, a drift region is divided into a plurality of long regions each interposed between a plurality of trenches, and a depletion layer expands in a drift region from an interface between the drift region and each of the trenches when a reversed bias is applied. Thus, the impurity concentration of the drift region can be normally heightened to cause a high breakdown voltage. Consequently, the on-resistance can be lowered by increasing the impurity concentration of the drift region.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.